Dementia research has become increasingly focused on the prodromal stages of Alzheimer's disease (AD). Elucidating the mechanisms of disease pathogenesis and the reliable identification of people in early stages of cognitive decline will be critical to the development of efficacious therapies for AD. Recently, a clinical entity termed mild cognitive impairment (MCI) has been recognized as a transitional state between the cognitive changes seen in normal aging and AD. The prevalence of MCI is more than double that of dementia and in many cases MCI may be prodromal AD. Therefore, MCI is a suitable condition for exploring the neurobiology underlying AD pathogenesis. There is a paucity of data on pathological and biological markers that distinguish individuals with MCI from healthy aged individuals. To investigate pathobiological markers for MCI, we will employ a novel two-tiered, discovery-based proteomics approach. First, we will use two-dimensional gel electrophoresis (2DGE) and liquid chromatography-tandem mass spectrometry (LCMS/ MS) to identify specific alterations in protein levels in the entorhinal cortex (EC) of subjects with no cognitive impairment (NCI), MCI, or early stage/mild AD. The EC is the initial site of degeneration in MCI and is an ideal area to explore MCI pathogenesis. Second, we will use new surface-enhanced laser desorption/ionization-time of flight MS (SELDI-TOF MS) protein chip technology to evaluate the changes in the cerebrospinal fluid (CSF) of these same individuals. Both tissue and fluid samples will be obtained from our ongoing NIA-funded longitudinal study of aging and AD, the Religious Orders Study (ROS). The proposed studies will hopefully lead to the discovery of new proteins involved in the pathogenesis of AD and new biomarkers for the timely diagnosis of persons in the earliest stages of this devastating disease. Both of these factors will be critical for the development of neuroprotective strategies for slowing or preventing cognitive decline in the elderly.